The people of OpenMoko know about the problem and are working on it. A [http://lists.openmoko.org/pipermail/openmoko-kernel/2008-July/003777.html software workaround] has been released as a kernel patch and may be acquired simply by doing an "opkg update" and "opkg upgrade" and then rebooting your FreeRunner. This workaround effectively turns off the SD card clock when the SD card is not being used. One will be able to use the SD card at the same time as the GPS except for the first approximately 30 seconds.

+

A [http://lists.openmoko.org/pipermail/openmoko-kernel/2008-July/003777.html software workaround] has been released as a kernel patch and may be acquired simply by doing an "opkg update" and "opkg upgrade" and then rebooting your FreeRunner. This workaround effectively turns off the SD card clock when the SD card is not being used. One will be able to use the SD card at the same time as the GPS except for the first approximately 30 seconds.

OpenMoko is also examining a hardware solution: soldering a 10 pf capacitor between SD card pin 5 (clk) and pin 6 (Vss) [[Image:Gta02 gps 10pf rework sop.pdf]]. This effectively restores GPS performance to the levels obtained without the SD card inside. The rework is not suggested to end users without hardware soldering technique and equipment.

OpenMoko is also examining a hardware solution: soldering a 10 pf capacitor between SD card pin 5 (clk) and pin 6 (Vss) [[Image:Gta02 gps 10pf rework sop.pdf]]. This effectively restores GPS performance to the levels obtained without the SD card inside. The rework is not suggested to end users without hardware soldering technique and equipment.

Revision as of 08:28, 24 July 2008

See GPS for details on how to evaluate the Freerunner GPS with command line utilities.

Contents

Problem: it takes a long time (10min+) before the Freerunner gets first GPS data

It was found that the OpenMoko FreeRunner GPS has a long Time To First Fix (TTFF) with the SD card in the phone. This thread indicates that there is interference from the SD card slot, as without the SD card inserted it the TTFF are much shorter.

More specific: the GPS signal level drops by -20dBm, i.e. factor 100. Signal strength of a GPS satellite above your head is around -127dBm. Needed strength for a first fix is -145dBm with a minimum of three satellites. Once the GPS chip has a fix, it can operate at signal levels of -157dBm, thus making it possible to operate while there is data transfer from/to the SD card.

Solutions

A software workaround has been released as a kernel patch and may be acquired simply by doing an "opkg update" and "opkg upgrade" and then rebooting your FreeRunner. This workaround effectively turns off the SD card clock when the SD card is not being used. One will be able to use the SD card at the same time as the GPS except for the first approximately 30 seconds.

OpenMoko is also examining a hardware solution: soldering a 10 pf capacitor between SD card pin 5 (clk) and pin 6 (Vss) File:Gta02 gps 10pf rework sop.pdf. This effectively restores GPS performance to the levels obtained without the SD card inside. The rework is not suggested to end users without hardware soldering technique and equipment.

Hardware mod related

See:

Speedevil's Shop for a UK service to perform this operation on your freerunner, or globally shipped 10pF capacitor.

Discussion

The following is based on Alasal's post to the community mailing list.

What is TTFF?

TTFF means Time To First Fix of the GPS. This is the time the GPS needs to get the first clue on where you are on planet earth. So you have to do this only once. After you have a FF (first fix), you can get more fixes without any problems with the SD card in the phone.

So if we have a first fix, the SD card doesn't block the GPS anymore?

Yes, the SD card doesn't block the GPS if we have a first fix. (Some people even claim it's more stable)

Why do we need a first fix?

According to Al Johnson, the Antares4 chip on the GTA02 doesn't have the memory needed to store almanac and ephemeris, last known position or time. This means that every start is a true cold start. It is possible to save location, almanac and ephemeris at GPS shut down, and restore these information at the next startup, but that does not seem implemented yet.

Does the lower signal also affect accuracy?

It is a good deal more jittery. However, if you drive around in a city you will get a lot more error from signal reflections than from the jitter.

Wrong, you will be able to read the SD card when the GPS is on. The clock for the SD card used to run constantly. Now the kernel turns it off when the SD card is not in use. As long as you are not constantly reading from the SD card, you should see much better first fix times. Obtaining a GPS fix does not affect SD performance.

Did Openmoko test the GPS with a SD card in it?

Yes.

Why didn't they find it while testing the Freerunner?

Because they have to do the tests with a fake GPS signal and with that signal it worked (In a fab you don't have a decent GPS signal). They have already modified the test, so it corresponds to the GPS signal better.

Information about faulty antennas

If removing the SD card doesn't help and you suspect your antenna, try unplugging and replugging the internal GPS connector. (See Disassembling_Neo1973 about 2/3 of the way down. (this may differ on freerunner, pictures appreciated)

Views

Personal tools

See GPS for details on how to evaluate the Freerunner GPS with command line utilities.

Problem: it takes a long time (10min+) before the Freerunner gets first GPS data

It was found that the OpenMoko FreeRunner GPS has a long Time To First Fix (TTFF) with the SD card in the phone. This thread indicates that there is interference from the SD card slot, as without the SD card inserted it the TTFF are much shorter.

More specific: the GPS signal level drops by -20dBm, i.e. factor 100. Signal strength of a GPS satellite above your head is around -127dBm. Needed strength for a first fix is -145dBm with a minimum of three satellites. Once the GPS chip has a fix, it can operate at signal levels of -157dBm, thus making it possible to operate while there is data transfer from/to the SD card.

Solutions

The people of OpenMoko know about the problem and are working on it. A software workaround has been released as a kernel patch and may be acquired simply by doing an "opkg update" and "opkg upgrade" and then rebooting your FreeRunner. This workaround effectively turns off the SD card clock when the SD card is not being used. One will be able to use the SD card at the same time as the GPS except for the first approximately 30 seconds.

OpenMoko is also examining a hardware solution: soldering a 10 pf capacitor between SD card pin 5 (clk) and pin 6 (Vss) File:Gta02 gps 10pf rework sop.pdf. This effectively restores GPS performance to the levels obtained without the SD card inside. The rework is not suggested to end users without hardware soldering technique and equipment.

Hardware mod related

See:

Speedevil's Shop for a UK service to perform this operation on your freerunner, or globally shipped 10pF capacitor.

Discussion

The following is based on Alasal's post to the community mailing list.

What is TTFF?

TTFF means Time To First Fix of the GPS. This is the time the GPS needs to get the first clue on where you are on planet earth. So you have to do this only once. After you have a FF (first fix), you can get more fixes without any problems with the SD card in the phone.

So if we have a first fix, the SD card doesn't block the GPS anymore?

Yes, the SD card doesn't block the GPS if we have a first fix. (Some people even claim it's more stable)

Why do we need a first fix?

According to Al Johnson, the Antares4 chip on the GTA02 doesn't have the memory needed to store almanac and ephemeris, last known position or time. This means that every start is a true cold start. It is possible to save location, almanac and ephemeris at GPS shut down, and restore these information at the next startup, but that does not seem implemented yet.

Does the lower signal also affect accuracy?

It is a good deal more jittery. However, if you drive around in a city you will get a lot more error from signal reflections than from the jitter.

Wrong, you will be able to read the SD card when the GPS is on. The clock for the SD card used to run constantly. Now the kernel turns it off when the SD card is not in use. As long as you are not constantly reading from the SD card, you should see much better first fix times. Obtaining a GPS fix does not affect SD performance.

Did Openmoko test the GPS with a SD card in it?

Yes.

Why didn't they find it while testing the Freerunner?

Because they have to do the tests with a fake GPS signal and with that signal it worked (In a fab you don't have a decent GPS signal). They have already modified the test, so it corresponds to the GPS signal better.

Information about faulty antennas

If removing the SD card doesn't help and you suspect your antenna, try unplugging and replugging the internal GPS connector. (See Disassembling_Neo1973 about 2/3 of the way down. (this may differ on freerunner, pictures appreciated)